Nutrient Cycling

The largest mass of nutrients is delivered by incoming tidal waters and, for example, a square meter of marsh inundated twice a day by 25 cm of tidal water carrying 0.5 mg dissolved inorganic N • L-1 has a potential supply of 250 mg N • m-2 • d-1. At least a portion of this potential N supply is retained within some marshes since ebb-tide concentrations of nitrate were roughly 1 mg NO3 • L-1, lower than mean nitrate concentrations in the mainstem, where NO3 is 2.5 mg NO3 • L-1 (Lampman, Caraco, and Cole, 1999). Phosphorus (P) concentrations in ebb-tide waters range from 40 to 80 |g PO4 • L-1 and are not demonstrably lower than in the main-stem (PO4 ~ 60 |g• L-1; Lampman et al., 1999). Given the significant plant demand for PO4, it suggests there must be reasonably large sources of PO4 to balance plant removal and maintain rough equality in flood versus ebb tide concentrations. This P source may be from tributary inputs since many tributaries (for example, Stony Creek and Saw Kill in the Tivoli Bays) receive treated sewage leading to greatly elevated P concentrations. Sediment P release is another likely candidate for this source of P since porewater concentrations are quite high (see below). If porewaters can bypass the surface oxidized layer where P is effectively scavenged (Vernon, 2002), the large reservoir of inorganic P in deeper sediment layers (ultimately derived from decay of organic materials) may contribute to the net efflux of P from Hudson marshes. There is essentially no information on sources or exchanges of organic nutrients (dissolved organic N, dissolved organic P, etc.) although some Hudson marshes act as minor sources of dissolved organic C to the mainstem (Findlay et al., 1998).

Tributary inputs of nutrients to Hudson River marshes vary greatly among sites due to both variation in size of tributary relative to area of marsh and variation in nutrient concentration, the latter of which seems related to agricultural lands and human population (Parsons and Lovett, 1992; Nieder, unpublished). Tributaries are only occasionally a large source compared to tidal waters; for instance, at Tivoli North Bay, Stony Creek has a relatively high dissolved inorganic N concentration (1998 mean >2 mg NO3 • L-1; Hudson River National Estuarine Research Reserve, unpublished data) and a modest discharge (~100 • L s-1 summer) resulting in a potential contribution of about 40 mg N • m-2 d-1 (assuming an intertidal marsh area of 1,000,000 m2 based on area of cattail plus purple loosestrife), much less than the potential tidal water contribution (250 mgN • m2 • d).